FIG. 1 shows a prior-art cryostat shown, for example, in European Patent Application No. 0,260,036 A2, published Mar. 16, 1688. It comprises a cryogen container 1 accommodating a cooled object, such as coils of a superconducting magnet, not shown, and containing a liquid cryogen, such as a liquid helium kept at a temperature of 4.2 K, with the cooled object being immersed in the liquid cryogen.
The device further comprises a refrigerator 2, a first heat shield 4 comprising walls surrounding the cryogen container 1, a second heat shield 5 comprising walls disposed between the cryogen container 1 and the first heat shield 4 and surrounding the cryogen container 1. The first and the second heat shields 4 and 5 shut off heat radiation from the outside to the cryogen container 1 and conduct heat to the refrigerator 2. The first and the second heat shields 4 and 5 constitute a cooled member 3 to be cooled by the refrigerator 2.
A vacuum container 6 accommodates the cryogen container 1, and the first and second heat shields 4 and 5, and its interior is kept at a vacuum state, thereby to provide a vacuum heat insulation.
The refrigerator 2 has a main block 2a situated outside the vacuum container 6, a first elongated, e.g., cylindrical, part 2b having a first end (or upper end as seen in FIG. 2) connected to the main block 2a and extending from the main block 2a, downward as seen in FIG. 1 thereby extending into the vacuum container 6 and having its tip or second end (lower end as seen in FIG. 2) situated near the first heat shield 4, and a second elongated, e.g., cylindrical, part 2c of a smaller diameter than the first cylindrical part 2b , being coaxial with the first cylindrical part 2b, having a first end (upper end as seen in FIG. 2) connected to the second end of the first cylindrical part 2b, and extending through an opening 4a provided in the first heat shield 4 and having its tip or second end (lower end as seen in FIG. 2) situated near the second heat shield 5.
A flange 7 is provided for mounting the refrigerator 2 to the vacuum container 6. Bellows 8 is provided between the vacuum container 6 and the flange 7 for absorbing any oscillation of the refrigerator 2 thereby preventing the oscillation from being transmitted to the vacuum container 6.
The refrigerator 2 has a first-stage cooling section 9 and a second-stage cooling section 14. The first-stage cooling section 9 is annular and is disposed to encircle the first cylindrical part 2b, in the vicinity of the refrigerator 2. The first-stage cooling section 9 comprises a collar part 9a having its inner periphery in contact with and fixed to the outer cylindrical surface of the first cylindrical part 2b, and a flange part 9b having its inner edge connected to the lower edge of the collar part 9a.
A first thermal coupling member 10 is flange-shaped and is in contact with the lower surface of the flange part 9b of the first-stage cooling section 9 for thermal conduction between them.
A first heat conduction plate 11 is in contact with the first thermal coupling member 10, and a first flexible conductive member 12 is in contact with the first heat conduction plate 11 and also with the first heat shield 4. The first thermal coupling member 10, the first heat conduction plate 11 and the first flexible conductive member 12 together provide thermal coupling between the first-stage cooling section 9 and the first heat shield 4. The first flexible conductive member 12 absorbs any thermal contraction due to temperature change.
A first jacket 13 has a cylindrical part 13a which is provided to surround the first-stage cooling section 9 and the first thermal coupling member 10. The first jacket 13 also has a flange-shaped part 13b having its outer edge connected to the lower end of the cylindrical part 13a. The space inside the first jacket 13 is filled with helium gas.
The second-stage cooling section 14 is annular and is mounted around the periphery of the tip (second end) of the second cylindrical part 2c. A disc-shaped second thermal coupling member 15 is in contact with the second-stage cooling section 14 for thermal conduction. A second thermal conduction plate 16 is in contact with the second-stage cooling section 14, and a second flexible conductive member 17 is in contact with the second heat shield 5. The second thermal coupling member 15, the second thermal conduction plate 16 and the second flexible conductive member 17 together provide thermal coupling between the second-stage cooling section 14 with the second heat shield 5. The second flexible conductive member 17 absorbs any thermal contraction due to temperature change.
A second jacket 18 has a cylindrical part 18a which is provided to surround the second cooling section 14 and the second thermal connecting member 15. The upper end of the cylindrical part 18a is connected to the inner edge of the flange-shaped part 13b. The second jacket 13 also has a disc-shaped part 18b having its peripheral edge connected to the lower end of the cylindrical part 18a. The space inside the second jacket 18 is filled with helium gas.
A compressor unit 19 supplies compressed helium gas to the refrigerator 2, and supplies electric power to a valve driving motor, not shown, built in the refrigerator 2.
The operation will now be described. The amount of heat infiltrating into the cryogen container 1 varies depending on the tempertures of the first and second heat shields 4 and 5. As the temperatures of the heat shields 4 and 5 are lower, the infiltration of heat is reduced, so is the consumption of the liquid helium cooling the coils of the superconducting magnet, or any other cooled object, accommodated in the cryogen container 1. Accordingly, the first and second heat shields 4 and 5 are cooled by the use of the refrigerator 2 to reduce the consumption of the liquid helium.
When the refrigerator 2 is made to operate, the first-stage cooling section 9 and the second-stage cooling section 14 are cooled to about 80 K and about 20 K, respectively and as a result, the first heat shield 4 is cooled via the first thermal coupling section 10, the flange-shaped part 13b, the first thermal conduction plate 11 and the first flexible conductive member 12, and the second heat shield 5 is cooled via the second thermal coupling section 15, the disc-shaped part 18b, the second thermal conduction plate 16 and the second flexible conductive member 17.
The refrigerator 2 (including the first and the second cylindrical parts 2b and 2c) sometimes needs to be removed for replacement or repair. For the removable, the first-stage cooling section 9 and the second-stage cooling section 14 are formed so that they can be separated from the first thermal coupling section 10 and the second thermal coupling section 15, respectively.
A problem associated with the above-described prior-art cryostat is that the contact pressure between the thermal coupling section 10 and 15 and the cooling sections 9 and 14 can vary depending on the manufacturing dimensional variations (within tolerances) and the thermal contraction of the cooling sections 9 and 14 and the jackets 13 and 18 surrounding the cooling sections 9 and 14, and the thermal conductivity at the contacting surfaces can be lowered.
Another problem associated with the prior-art cryostat is that when it is tiled by 90.degree. from the state shown in FIG. 1, due to helium gas convection, heat is conducted from the part closer to the outside of the vacuum container 6 to the part closer to the cryogen container 1, and cooling effect is degraded.
A further problem is that as the refrigerator 2 is operated, the pressure inside the first jacket 13 becomes negative (lower than the atmospheric pressure), and air may leak into the space inside the first jacket 13, through a sealing part, not specifically indicated, and may be frozen in the space inside the jacket 13.
A yet further problem is that when the refrigerator is removed for replacement or for repair, it is necessary to cover the mounting part at which the refrigerator 2 is mounted with a gas bag, not shown, and fill the gas bag with helium gas, before actually removing the refrigerator. Such work is time-consuming.